1. Field of the Invention
The present invention relates to a method and system for controlling a magnetic resonance signal acquisition sequence.
2. Description of the Related Art
In a conventional medical MRI (magnetic resonance imaging) apparatus, gradient fields and radio frequency (RF) pulses are applied to a subject placed in a static field to occur a magnetic resonance (MR) phenomenon in a specific portion of the patient. An MR signal (echo signal) induced by the MR phenomenon is detected and processed to acquire anatomical data or the like.
Data obtained by the MRI apparatus reflects parameters such as an atomic nuclear density (a hydrogen atomic nuclear density will be referred to as "PD" hereinafter) of the excited specific portion, a longitudinal relaxation time T1, a transverse relaxation time T2, and the like. A conventional pulse sequence used for acquiring MR images having different emphasis for three parameters PD, T1, and T2 will be described below with reference to FIGS. 1 and 2.
FIG. 1 shows a multi-echo signal acquisition pulse sequence using a spin echo (SE) method. With this pulse sequence, MR images in which the hydrogen atomic nuclear density PD and the transverse relaxation time T2 are emphasized can be acquired. More specifically, after excitation of a specific portion of a subject upon application of an RF pulse having a flip angle of 90.degree. (90.degree. pulse or excitation pulse) and a slice gradient field (G.sub.S), a spin phase is converged upon application of an RF pulse having a flip angle of 180.degree. (180.degree. pulse or inversion pulse). After the lapse of a predetermined echo time TE1 from the application of the 90.degree. pulse, a first MR signal is acquired. After acquisition of the first MR signal, 180.degree. pulses are sequentially applied to converge the spin phase, thus acquiring a second MR signal, a third MR signal, and so on.
In this pulse sequence, when the echo time TE1 is set to be shorter than a normal time, a first MR signal with the emphasized hydrogen atomic nuclear density PD can be obtained. When an echo time TE2 is set to be longer than a normal time, a second MR signal with the emphasized transverse relaxation time T2 can be obtained. The echo time TE2 corresponds to an integer multiple of the echo time TE1. For example, the echo time TE1 is set to be 30 msec, and the echo time TE2 is set to be 60 msec.
In order to acquire MR signals with the further emphasized hydrogen atomic nuclear density PD and transverse relaxation time T2, a variable echo method for changing an echo time is used. For example, the echo time TE1 is set to be 20 msec, and the echo time TE2 is set to be 120 msec.
FIG. 2 shows an echo signal acquisition pulse sequence using a gradient field echo (FE) method. When a short echo time is set, an MR signal with the emphasized hydrogen atomic nuclear density PD can be acquired. The difference between the gradient FE method and the pulse echo method is that an inverted read gradient field is applied in place of 180.degree. pulses so as to converge the spin phase.
When the pulse sequence is executed using the gradient FE method and the pulse echo method, MR signals with the emphasized hydrogen atomic nuclear density PD and transverse relaxation time T2 can be obtained. However, it is difficult to acquire an MR signal with the emphasized longitudinal relaxation time T1.
In this manner, when the pulse sequence is executed once by the conventional method, MR images in which two out of three parameters, i.e., the hydrogen atomic nuclear density PD, the longitudinal relaxation time T1, and the transverse relaxation time T2 are respectively emphasized can be acquired. However, MR images in which the three parameters are respectively emphasized cannot be obtained. Therefore, in order to acquire MR images in which the three parameters are respectively emphasized, at least two pulse sequences must be executed.
Thus, a demand has arisen for the MRI apparatus capable of acquiring MR images in which the hydrogen atomic nuclear density PD, the longitudinal relaxation time T1, and the transverse relaxation time T2 are respectively emphasized in a single sequence.